In the production of rubber articles such as hose, pneumatic tires or power transmission belts such as V-belts, toothed positive drive belts, etc., it is generally necessary to reinforce the rubber or elastomeric product. In the past, textile materials have been primarily employed for this purpose. However, wire cord has been found to be more desirable under certain conditions of use, for example, in pneumatic tires of the radial ply type. Maximum reinforcement of the rubber is obtained when maximum adhesion is produced and retained between the laminate of a contiguous rubber skim stock and the metal reinforcing element as used to form a unitary structure. Of equal importance is the requirement that, for example, the laminate of the reinforcing metal element and contiguous rubber remain in a bonded relationship with each other throughout the useful life of the reinforced structure in which the laminate is used.
In order to promote adhesion between rubber and ferrous metals it is known to employ a variety of metallic salts as coatings to the metal or as an ingredient in a rubber composition. One early patent embodying the former technique is U.S. Pat. No. 1,919,718 which discloses a composite rubber product that adheres to ferrous metals. The invention requires the coating of the metal with a metallic salt such as that formed with copper, cobalt, manganese, iron or lead and a fatty acid of high molecular weight, e.g., stearic, palmitic, oleic, linoleic, lauric and the like. Alternatively, the metallic salt may be dissolved in a solvent or in a rubber cement which is then applied to the metal surface. The rubber layer is then applied and vulcanized in position.
U.S. Pat. No. 2,643,273 discloses a rubber adhesive composition which contains a water soluble cobalt salt to increase the adherence of rubber to metal. The specification discloses incorporating cobalt chloride or sulfate in the rubber or dipping the metal in a solution of the salt.
U.S. Pat. No. 2,912,355 is directed toward improving the adhesion between rubber and metal by the incorporation into a rubber composition of a partially oxidized metal salt of an aliphatic fatty acid compound, the metal being cobalt, copper, iron, lead, mercury, nickel or silver.
U.S. Pat. No. 3,897,583 is directed toward the adhesion of metal to rubber by incorporating a cobalt salt in a rubber stock which contains an adhesive resin forming system based on a methylene donor which is a methylolated nitroalkane in combination with a resorcinol type methylene acceptor. Cobalt salts disclosed include those of aliphatic or alicyclic carboxylic acids having 6-30 carbon atoms.
U.S. Pat. No. 3,903,026 discloses a rubber composition containing cobalt carboxylate and magnesium oxide to improve the adhesion of the rubber to zinc or zinc alloy plated steel.
U.S. Pat. No. 3,936,536 discloses the method of adhering rubber to metal by coating the metal surface with rubber containing a small amount of Co(NH.sub.3).sub.2.Cl.sub.2.
U.S. Pat. No. 3,993,847 discloses a process of bonding a rubber to a ferrous metal and such a laminated article. Adhesion is improved by employing an adhesive comprising a conjugated diene and a heterocyclic nitrogen base and containing about 5-180 parts of silica filler per 100 parts of the diene-heterocyclic nitrogen interpolymer. The patent states that adhesion is further improved by using a rubber containing small amounts of cobalt metal.
While others have sought to enhance adhesion between rubber compositions and metals by employing certain metallic salts and complexes with or without other compounds such as resins, the art of which we are aware, has not disclosed a satisfactory metal salt that can complement the presence of a cobalt salt when amounts of the latter are decreased and yet maintain adhesion properties as good as when the cobalt salt is utilized as the sole metal compound to improve rubber-to-metal adhesion.
U.S. Pat. No. 3,998,992 to Nakamura is directed to the replacement of a part of the conventionally used carbon black filler with finely divided high purity silica in rubber compositions to improve rubber-ferrous metal adhesion. Nakamura '992 describes the background art as including rubber-brass adhesion promoters such as cobalt naphthenate, again, an organo-cobalt compound. Nakamura '992 also discloses the use of cobalt salts of organic acids as processing aids and resin acids as accelerator activators for rubber stock to be used in tires. The use of either as independent adhesion promoters are not suggested, however.
U.S. Pat. No. 4,057,529 to Leo, et al., discloses improved rubber-metal adhesion with tire cord resulting from the compounding of rubber with a magnesium compound and a carboxylated cobalt oxy metal complex, such as carboxylated cobalt borate (0.5 to 2.5 phr). Again, the cobalt compound disclosed is an organo-cobalt compound.
U.S. Pat. No. 3,991,130 discloses the use of organo-nickel salts as adhesion promoters for vulcanizable elastomers to metals. These salts are, in fact, complexes wherein two organic ligands complex with nickel to form an organo-nickel salt.
To achieve satisfactory rubber-to-metal bonding, numerous methods have been developed. For example, U.S. Pat. No. 2,720,479 describes a system wherein a phenolic resin and a brominated isoolefin-polyolefin interpolymer are dissolved in a suitable liquid carrier and the resulting adhesive composition is spread on rubber which is to be bonded to metal. The rubber and metal are subsequently pressed together and vulcanized. U.S. Pat. No. 2,581,920 also describes the use of halogenated polymers to bond rubber to metal.
U.S. Pat. No. 2,581,920 to Kuhn describes a process of improving metal-to-rubber adhesion by the use of a blend of dichlorobutadiene (2,3-dichlorobutadiene-1,3) and certain chlorinated resins; this process is preferably practiced by coating the metallic substrate with a volatile solvent solution of the adhesive composition.